Ok, I changed my diff fluid for the first time yesterday, as well as the t-case fluid. I was surprised at how good the fluid coming out of the diffs looked compared to the t-case. The fluid in the diffs looked almost brand new, while the t-case looked a little like used motor oil. I replaced all 3 with Mystik JT7 80W-90, and WOW, what a difference! I had been having some intermittent drive line noises, and those are now gone. Also, I had been running about 2250 RPM at 60 MPH, and now I'm running between 2000 and 2250 RPM at 60 MPH. May not make a huge difference, but we'll see how the MPG numbers work out over the next few tanks of gas.

 

The fluid change has nothing to do with a change in your RPM's.
Not knowing the mileage on your truck, I would be concerned with the old t/case fluid and your noise.
I would change it again real soon and pull the inspection cover to check out the internal gears then refill with an 85W/140 for extra wear protection.
What about the rest of your trucks services, have you done a major recently if not, look at the major service sticky at the top of this section for a guide.

 

here's the write up on plate remove / re-seal for transfer case.

D1 Transfer Case Fluid Change with Pictures - Land Rover Forums - Land Rover Enthusiast Forum

The hand pump that goes on the oil jug is a great thing to have.

 

If it is like most others who had their truck serviced at the quick lube before you bought it the t-case fluid has never been changed because those guys dont know where the drain and fill plugs are.
I wouldn't worry about it, but thats just me, I dont worry about much if anything, to busy having fun and living life.